Conventionally, disassembling a wheel from the axle thereof is done either by human power or pneumatic power. A pneumatic device generally requires a power source which is not easy to carry in a passenger car. Using hand tools to disassemble, although convenient to be carried, is difficult and takes great effort. To overcome the difficult, U.S. Pat. No. 4,727,780 disclosed by the inventor of the present inventor provides a handy electrical wrench which generates impact torque and applies this torque to an over-tight stud screw of a wheel so as to loose the screw which in general may not be easy to loose by human power only. The wrench is small in size and compact in design and thus is as easy to carry as a hand tool.
The inventor of the present invention further disclosed an improvement over the above-mentioned patent, which is a continuation-in-part of the above-mentioned patent and has been assigned the U.S. Pat. No. 4,920,831. In the U.S. Pat. No. 4,920,831, and arrestor is added to the electrical wrench. The arrestor provides an automatic engagement/disengagement of power trassmision to the stud screw while the work is done manually in the U.S. Pat. No. 4,727,780.
The devices disclosed in these patents, however, have a disadvantage. That is, when torque is to be transmitted to the stud screws, a moving part thereof has to engage with other parts so as to establish the transmission; and this is not done very smoothly in the devices of the previous patents. Since the transmitted torque is great, the un-smooth engagement usually results in a damage to the device after a long while operation.
It is therefore an object of the invention to improve the devices and to obviate the drawback.